1. Field of the Invention
The present invention relates to a face detecting method for detecting face of human subjects within images, and a photography apparatus that employs the face detecting method.
2. Description of the Related Art
The basic principle of face detection, for example, is classification into two classes, either a class of faces or a class not of faces. A technique called “boosting” is commonly used as a classification method for classifying faces (refer to, for example, U.S. Pat. No. 5,819,247). The boosting algorithm is a learning method for classifiers that links a plurality of weak classifiers to form a single strong classifier.
U.S. Patent Application Publication No. 20020102024 discloses a method that speeds up face detecting processes by the boosting technique. In this method, the weak classifiers are provided in a cascade structure, and only images which have been judged to represent faces by upstream weak classifiers are subject to judgment by downstream weak classifiers.
Face detection employing the boosting algorithm such as that disclosed in U.S. Pat. No. 5,819,247 generally employs brightness data (Y signals) of images. On the other hand, image data obtained by digital cameras and the like often are RGB signals. Accordingly, when face detecting processes are administered onto images obtained by digital cameras and the like, it becomes necessary to generate Y signals from the RGB signals. This increases the amount of data to be processed, and prevents face detection from being expedient. Particularly in cases that face detecting apparatuses are incorporated into digital cameras and face detecting processes are administered onto images immediately after obtainment thereof, the Y signal generating process becomes a large factor in preventing fast face detection.
In addition, there are many cases that natural sunlight or flash are used as illumination during photography employing photography apparatuses such as digital cameras. However, there are other cases in which tungsten light sources are used as illumination during photography of buildings, nightscapes, and products. Tungsten light sources have a property that the light emitted therefrom contain more red components than other light sources. Therefore, in the case that face detection is performed with respect to images obtained using tungsten light sources as illumination in the same manner as that performed with respect to images obtained using natural sunlight as illumination, there is a problem that faces cannot be detected therein.
Further, in cases that the brightness is low (dark) during photography, there is a problem that there is insufficient brightness within the obtained images. In these cases, the aforementioned boosting algorithm is not capable of detecting faces within the obtained images.